Recently, lithium ion secondary batteries (hereinafter referred to as simply “lithium secondary batteries”) have been increasingly used as the power source for portable electronic devices. They use, for example, a carbonaceous material capable of absorbing and desorbing lithium as the negative electrode active material, and use a composite oxide containing transition metal and lithium, such as LiCoO2 (lithium cobaltate), as the positive electrode active material. By this, secondary batteries with high potential and high discharge capacity are realized.
However, with the recent increase in functionality of electronic devices and communications devices, it is desired to further improve the performance of lithium secondary batteries, in particular, to reduce performance degradation due to charge/discharge cycles.
Generally, an electrode plate, which is a power generating element of a lithium secondary battery, is produced by mixing and dispersing a positive electrode active material or negative electrode active material, a binder as a binding agent, and optionally a conductive agent in a dispersion medium, applying the resulting paint mixture onto one face or both faces of a current collector, drying it, and pressing it to form a positive electrode mixture layer or negative electrode mixture layer.
One cause of performance degradation due to charge/discharge cycles is a decrease in the adhesion between the positive electrode mixture layer or negative electrode mixture layer applied onto the current collector and the current collector. This occurs because the expansion and contraction of the electrode plate due to charge/discharge decreases the adhesion of the positive electrode mixture layer or negative electrode mixture layer at the interface of the current collector, thereby causing it to separate therefrom.
To increase the adhesion between the positive electrode mixture layer or negative electrode mixture layer and the current collector, attempts have been made to increase the interfacial contact area of the current collector. In this case, the surface of a current collector is usually roughened by a method of etching the surface of a current collector by electrolysis or a method of depositing constituent metal on the surface by electrodeposition.
As another method to roughen the surface of a current collector, there has been proposed, for example, a method of causing fine particles to collide with the surface of a rolled copper foil, which is the material to be processed, at a high speed to form minute protrusions and depressions on the surface (see, for example, Patent Document 1). There has also been proposed a method of irradiating a metal foil with a laser beam to form protrusions and depressions whose surface roughness is 0.5 to 10 μm in 10-point average roughness (see, for example, Patent Document 2).
Also, reducing the thickness of the applied electrode mixture layer to enhance power density results in a decrease in energy density. To avoid this dilemma, it has been proposed to increase the surface area of a current collector in order to increase the contact area of the electrode mixture layer and the current collector, as illustrated in FIG. 27. In the example illustrated in this figure, a metal foil current collector 61 is sandwiched between a pair of guide rollers 62 and 63 to form protrusions and depressions on the surface thereof (see, for example, Patent Document 3).
Also, to obtain a current collector for a lithium secondary battery that can firmly retain an active material and has good electrical conductivity, it has been proposed, for example, to form a metal foil into the shape of a corrugated sheet having ridges and grooves, so that one face of the metal foil is recessed while the other face is raised, as illustrated in FIG. 28 (see, for example, Patent Document 4).
Further, to obtain an inexpensive, long-life lithium secondary battery having good power characteristics and little variations in capacity, power, etc., there has been proposed a method of forming depressions and protrusions by embossing, filling the depressions with an active material, and making the surfaces of the protrusions exposed or covered with the active material, as illustrated in FIG. 29 (see, for example, Patent Document 5).
Another known method for producing an electrode plate for a lithium secondary battery is a method of forming a thin film of an active material on a current collector by electrolytic plating, vacuum deposition, or the like. In this method, the adhesion between the current collector and the active material is also essential to obtaining a stable battery. For example, with the aim of obtaining an electrode plate for a lithium secondary battery having a large discharge capacity and excellent charge/discharge cycle characteristics, a method of forming an active material thin film on a current collector made of a metal not alloyable with lithium has been proposed. In this method, the value: (the surface roughness Ra of the active material thin film)−(the surface roughness Ra of the current collector) is set to 0.1 μm or less (see, for example, Patent Document 6).    Patent Document 1: Japanese Laid-Open Patent Publication No. 2002-79466    Patent Document 2: Japanese Laid-Open Patent Publication No. 2003-258182    Patent Document 3: Japanese Laid-Open Patent Publication No. Hei 8-195202    Patent Document 4: Japanese Laid-Open Patent Publication No. 2002-270186    Patent Document 5: Japanese Laid-Open Patent Publication No. 2005-32642    Patent Document 6: Japanese Laid-Open Patent Publication No. 2002-279972